Copper clad laminates are the basic component of the printed circuit boards used in the electronics industry. In the most common processes, a copper foil is bonded to a prepreg of resin, such as epoxy resin, impregnated fiberglass by heat and pressure. The copper foil surface that is pressed against the prepreg is typically an uneven or profiled surface with some kind of additional bonding treatment applied to insure that the laminate remains together under normal processing conditions.
The opposing foil surface (the surface not bonded to the prepreg) is typically a smooth surface with various treatments that are aimed at preventing oxidation of the foil and allowing solder wettability and adequate photoresist adhesion. The opposing foil surface is often scrubbed to remove various debris prior to applying the photoresist. In particular, resin dust formed during the handling of the prepreg during lay-up of the laminate has a tendency to land on the shiny surface of the copper foil to which it may become undesirably bonded during the lamination cycle. In fact, any organic material may become undesirably bonded to metal foil during the lamination cycle. If these resin dust spots are not removed prior to etching the laminate when producing a pattern of lines and spaces of copper, then the resin spots shield portions of the copper from the etching solution and prevent its complete removal. As a result, forming fine metal lines with laminates contaminated with resin dust is difficult. If a resin dust spot is large enough and in the wrong location, it provides an unintended bridge between copper lines that would cause a short in the subsequently formed circuits and render the board useless. Etching copper clad laminates with resin dust spots, especially epoxy resin dust spots, is thus problematic.
Recently two types of metal foil are available where scrubbing of the exposed laminate surface is not practical. The first type is double treat foil for multilayer boards, where both sides of the copper foil are given an additional bonding treatment because after the first lamination, another prepreg is placed against the opposing foil surface and pressed to form a prepreg-foil-prepreg "sandwich." Any scrubbing of this surface would damage the bonding treatment and render it less capable of forming a strong bond to the second prepreg.
The second type of metal foil is reverse treat foil, where the bonding treatment is applied only to the smooth side of the foil that, in turn, is laminated to the prepreg, leaving the uneven or matte surface exposed to the resin dust. Scrubbing of this surface is an undesirable process that reduces some of the profile desired for photoresist attachment and future processing. Scrubbing certain metal foil surfaces is disfavored.